1. Field of the Invention
The present invention relates to a method and system for generating power from low- and mid-temperature heat sources using a zeotropic mixture as a working fluid.
2. Description of the Related Art
The world is struggling to meet its energy demand and the extensive consumption of fossil fuels has increased concerns regarding the emission of greenhouse gases. Vast amounts of industrial waste heat, as well as renewable energies like solar, thermal, and geothermal have not been efficiently utilized because of their low energy density and low conversion efficiency. When gas, liquids, or solids that contain heat are discharged into the environment, not only is the energy wasted, it puts the environment in potential jeopardy. For this reason, different methods and processes for converting the aforesaid energy into usable forms are under study.
One option of utilizing low- and mid-temperature heat is to convert it into power. The traditional steam Rankine cycle is economical only when it is applied to the conversion of heat with temperatures higher than around 588K (600 F) or where there is a large overall heat content. In order to obtain greater compatibility with the low- and mid-temperature heat source streams, various organic working fluids as well as ammonia and carbon dioxide are suggested as a substitute to water (steam).
Both organic Rankine cycle and supercritical Rankine cycle have been proposed. In a supercritical Rankine cycle, instead of passing though the two phase region with a boiling system like in an organic Rankine cycle, a working fluid is heated directly from the liquid state into the supercritical state, which allows it to have a better thermal matching with the heat source than an organic Rankine cycle. Furthermore, a boiling system requires specialized equipment to separate the vapor phase from the liquid phase, and the supercritical Rankine cycle system has the potential of simplifying the cycle by omitting the boiling system. The concept of the supercritical Rankine cycle and the advantage of using supercritical conditions have been recognized for a long time. For example, U.S. Pat. No. 1,632,575 to Abendroth describes a system for generating power from supercritical steam. A combined supercritical steam cycle system is proposed in U.S. patent application Ser. No. 11/905,846 to Tomlinson et al. U.S. Pat. No. 3,683,621 to Szewalski discloses a method of improving the power cycle efficiency of a steam turbine for supercritical steam conditions. A supercritical cycle is also discussed in U.S. Pat. No. 4,142,108 to Matthews for geothermal energy conversion.
As much as the supercritical Rankine cycle is superior to a conventional Rankine cycle in many aspects, supercritical steam Rankine cycle cannot be used for the conversion of low- and mid-temperature heat due to its high critical temperature. The working fluid of a supercritical Rankine cycle is the key factor deciding its application and performance. Only a few working fluids have been proposed to be used in a supercritical Rankine cycle for low- and mid-temperature heat conversion. In U.S. Pat. No. 6,751,959 B1 to T. S. McClanahan, a single-stage supercritical Rankine cycle using ammonia as the working fluid is discussed. Carbon dioxide used as the working fluid in supercritical Rankine cycles is discussed in a number of patents (U.S. Pat. No. 3,971,211 to Wethe; U.S. Pat. No. 3,237,403 to Feher; U.S. Pat. No. 4,498,289 to Osgerby). U.S. Pat. No. 4,358,930 to Pope, claims a method of optimizing the performance of Rankine cycle power plants using supercritical hydrocarbon (or mixture of hydrocarbons) as the working fluid. U.S. Pat. No. 7,007,474 B1 to Ochs discusses a method of recovering energy from a supercritical fluid by inclemently expanding the supercritical fluid entering at least one of the expansion engines with a low quality heat source.
Outside of patent literature, a 2008 paper [Sotirios Karellas and Andreas Schuster, “Supercritical Fluid Parameters in Organic Rankine Cycle Applications”, Int. J. Thermodynamics—Vol. 11, No. 3, 2008, pp. 101-108] compares a supercritical Rankine cycle with a normal organic Rankine cycle using the same working fluids (R134a, R227ea, R236fa, R245fa) to find out that the total efficiency of the supercritical Rankine cycle is 10%-20% higher than that of the regular organic Rankine cycle. It was also described that “the investigation of supercritical parameters in ORC applications seems to bring promising results in decentralized energy production[.]”